A. Technical Field
This present invention relates to integrated micromechanical devices having movable parts and, more particularly, to system and methods of recovering from stiction conditions in micro-electro-mechanical system (MEMS) devices.
B. Background of the Invention
Stiction is an area of concern in MEMS type devices having movable parts. Stiction between two surfaces typically occurs when an external force deflects a movable part in a manner so as to cause a section of its surface to come in physical contact and adhere to a surface of an adjacent stationary part. Sensor type MEMS devices are particularly vulnerable to this intermittent phenomenon, which can be caused by a variety of forces and adversely affects device performance. Forces that tend to cause stiction include: 1) electrostatic forces resulting from a voltage bias or a residual charge present on a surface; 2) capillary forces due to the presence of moisture; and 3) adhesion forces due to surface contamination or process residuals.
For example, in a z-axis MEMS accelerometer, an external disturbance such as a mechanical shock may deflect a suspended proof mass in a manner so as to cause a portion of its surface to contact and adhere to an adjacent wafer substrate surface. When the total adhesion force between the two surfaces is higher than the mechanical restoring force inherent to the proof mass, stiction results and temporarily immobilizes the proof mass preventing it from recovering its original position even after the external disturbance ceases. This renders the device unusable until the stiction force is overcome by a sufficiently large counteracting force.
Since stiction causes the proof mass to adhere to the substrate, the two parts are no longer separated from each other, blocking the movement of the proof mass and, in some cases, also causing a short circuit event that destroys the electric field between the two surfaces. Therefore, the device can no longer measure capacitive changes to derive an acceleration value during the time the stiction condition is present, which affects both device reliability and performance.
Some prior art approaches allow to improve stiction robustness of a device, for example, by increasing material stiffness and, thus, mechanical restoring force in order to aid in releasing the adhered parts of the device. Other approaches improve surface conditions during the fabrication process in order to minimize stiction. However, such improvements result from design tradeoffs that come at the cost of reduced device performance, increased device size, or increased cost of manufacturing. What is needed are tools for MEMS designers to overcome the above-described limitations without increasing device size or sacrificing device performance.